The invention concerns a resettable device for protecting electrical circuits against overcurrents. It is more particularly concerned with a resettable self-protected electrical switch for low-voltage electrical power supplies.
Ordinary electrical devices such as motors, transformers, batteries, domestic appliances and electronic circuits can be protected by materials with a non-linear electrical resistance, such as zinc oxide in the case of overvoltages. Protection against overcurrents is conventionally provided by fuses, and is non-reversible, the fuse having to be changed after use, or can be provided by mechanical devices such as resettable circuit-breakers.
A main object of the invention is a new device for protection against overcurrents in an electrical circuit. A second aim of the invention is for the device to automatically become conductive again after the overcurrent has ceased. A very simple circuit with a low cost to manufacture constitute a third objective of the invention.
To this end, the invention proposes a device protecting against overcurrents in an electrical circuit between a voltage supply and a load downstream of said device, said device including means having a high current sensitivity and adapted to be tripped (i.e. to change from a conductive state to a virtually non-conductive state) fast, but reversibly, in the event of an overcurrent in the circuit, and second means connected in parallel with the first means, adapted to withstand a temporary voltage higher than the mains voltage when the first means trip and to trip quickly and reversibly after the latter.
Clearly, this arrangement separates the stresses of fast tripping in the presence of an overcurrent, on the one hand, and of transfer of voltage corresponding to the mains supply, i.e. 230 V, on the other hand, using two separate components mounted in parallel and operating one after the other in the event of tripping in response to an overcurrent.
The reversibility of tripping is naturally a necessity if the protection device must be resettable.
The tripping means are preferably resistors whose resistivity increases reversibly by several orders of magnitude with temperature, and the first component has a very low resistance when in the conductive state compared to the resistance of the second component when in the conductive state.
To be more precise, the tripping means are groups of positive temperature coefficient (PTC) charged conductive polymer components.
These components are available off the shelf at low cost and have a resistance that varies by several orders of magnitude when the internal temperature of the component increases a few tens of degrees, and can therefore be used in a circuit of the above kind. Their use enables the economic fabrication of overcurrent protection devices.
Also, in this case, resetting of the protection device is automatic. As soon as the temperature of the polymer components falls back to the normal ambient temperature, they become conductive again and the circuit is made.
The invention is also directed to an electronic switch including a triac including a protection device as defined above connected in series on the input side of the triac.
A switch of the above kind is protected against overcurrents, there remaining only a current of a few milliamperes after tripping of the two variable resistance polymer components. It can then be desirable to eliminate the residual current in the circuit completely, for safety reasons and to allow the polymer components to cool.
To this end, a third PTC conductive polymer component is preferably mounted between the trigger of the triac and the voltage supply, this third component having a high resistance when in the conductive state relative to the resistance of the first component when in the conductive state.
According to various features of the invention, which can possibly be used conjointly:
the resistance of the first component in the conductive state is approximately 200 milli-ohms and the resistance of the second component in the conductive state is approximately 10 ohms;
the first component has a holding current of a few amperes at 20xc2x0 C.
the second component has a holding current of approximately 0.1 ampere at 20xc2x0 C.
These features are favorable to the fabrication of a current protection device for a conventional switch using components available off the shelf at low cost.
The description and the drawing of a preferred embodiment of the invention given hereinafter explain the aims and advantages of the invention. Clearly the description is given by way of example only and is not limiting on the invention.